1. Field of the Invention
The present invention relates to the field of bioscaffoldings formed in an infarct region of the heart or bioscaffoldings used as a coating on biomedical device such as a stent or a pacemaker lead. In particular, the invention relates to biofunctionalized bioscaffoldings formed of hyaluronan crosslinked with another type of hydrogel that is naturally found in the extra-cellular matrix (ECM), such as collagen, collagen-laminin, and poly-l-lysine. Fibrinogen or alginate hydrogels may also be used in combination with hyaluronan.
2. Discussion of Related Art
Myocardial infarction (MI) is one form of heart disease that often results from the sudden lack of the supply of oxygen and other nutrients due to the lack of blood supply to a portion of the heart. The lack of blood supply is a result of the closure of the coronary artery that nourishes a part of the heart muscle in the left ventricle. The coronary artery 110 containing a blockage 120 is illustrated in FIG. 1a. The cause of this event in coronary vessels is generally caused by arteriosclerosis, the “hardening of the arteries.” MI may also be the result of minor blockages where, for example, there is a rupture of cholesterol plaque resulting in blood clotting within the artery. Thus, the flow of blood is blocked and downstream cellular damage occurs. As a result of this insult to the heart tissue, scar tissue tends to naturally form. FIGS. 1a and 1b illustrate the progression of heart damage once the build-up of plaque induces a blockage 120 to occur. FIG. 1a illustrates a site of a blockage 120 and the resulting restricted blood flow can occur from any of the indicated causes. FIG. 1b illustrates the extensive damage to the left ventricle that can be a result of the lack of oxygen and nutrient flow to the left ventricle of the heart 100. FIG. 1b illustrates the two regions of an infarct region. The infarct region has (1) the “necrotic zone” 130 which is a region of significant necrosis/apoptosis tissue and (2) the “border zone” 140 that consists of a large concentration of apoptotic and necrotic tissue as well as viable tissue. In the border zone the cells exist in an oxygen-deprived state due to the blockage 120 of the coronary artery 110. The region of the heart beyond the border zone 140 is the “remote zone” which is remote of the infarct region and of the damage.
The infarct area will likely undergo remodeling and will eventually form a scar, leading to an area of the heart that does not contract. The remodeling of the heart is due to mechanical forces resulting in uneven stress and strain distribution in the left ventricle. MI damage can cause irregular rhythms of the heart that can be fatal, even though the remaining muscle is strong enough to pump a sufficient amount of blood. Remodeling of the heart begins immediately after an MI. The principle components of the remodeling event include myocyte death, edema and inflammation, followed by fibroblast infiltration, collagen deposition, and finally scar formation. The principle component of the scar is collagen. Because mature myocytes of an adult are not regenerated, the infarct region experiences significant thinning, as illustrated in FIG. 2b. FIG. 2a illustrates a normal cross-section of a wall of the left ventricle 210. FIG. 2b illustrates the thinning of the wall 220 after an MI. During systole (contraction of the left ventricle), due to the remodeling of the region, the infarct region may not move very much at all as illustrated in FIG. 2b. The remodeling of the region may also cause the wall 220 to bulge out as illustrated in FIG. 2c, and in an extreme case, rupture.
Over time, post-MI morphological changes occur. The gross morphological changes that occur over approximately a 7-week period are pallor of the myocardium that leads to some hyperemia, and then a yellowing starts to occur central to the damaged region. At approximately 15 days, the area is mostly yellow with soft vascular margins. This area eventually turns white from fibrosis. On a microscopic level, the initial examination reveals wavy myocardial fibers. Coagulation and necrosis with loss of cross striations occur followed by contraction bands, edema, hemorrhage, and neutrophilic infiltrate. Within 24-72 days there is total loss of nuclei and striations and heavy neutrophilic infiltrate. Then macrophage and mononuclear infiltration begin resulting in a fibrovascular response. Once this fibrovascular response occurs, prominent granulation of the tissue follows. This ultimately leads to fibrosis and a scar is formed by about 7 weeks post MI. This timeline illustrates the importance of regenerating the infarct region within a time before extensive scarring and damage occurs.
Despite recent advances in the treatment of acute myocardial infarction (MI), the ability to repair extensive myocardial damage and to treat heart failure is limited. The myocardium is unable to regenerate because there are insufficient numbers of cardiomyocytes or because the cardiomyocytes cannot replicate after injury and because there apparently are no muscle stem cells in the myocardium. The damage of MI is often progressive. Patients who survive MI are prone to scar tissue formation and aneurismal thinning of the damaged region. Even in the absence of cardiac aneurysm, the loss of viable myocardium can result in increased wall stress in the remaining myocardium, eventually triggering a sequence of molecular, cellular, and physiological responses that lead to left ventricular (LV) dilatation and heart failure.
In most cases, cardiac transplantation is the only available treatment that significantly lengthens and improves quality of life. It is limited, however, due to a chronic shortage of donor hearts. A possible strategy to restore heart function after myocardial injury is to induce angiogenesis through the use of a scaffolding. The scaffold temporarily provides the biomechanical support for the cells until they produce their own extracellular matrix and the scaffold may also lower stress in the infarct region by bulking up the region. Because the scaffolding may contain or attract living cells, they may have the potential to induce angiogenesis. Angiogenesis is the growth of new capillaries in the region. After an MI, the infarct tissue as well as the border zone and the remote zone begin to remodel. The scar tissue forms in the infarct region as the granulation is replaced with collagen, causing the scar to thin out and stretch. The perfusion (oxygen flow) in this region is typically 10% of the healthy zone, decreasing the number of active capillaries and therefore limiting the amount of angiogenesis that may occur. Increasing the number of capillaries may lead to an increase in angiogenesis. Other benefits of increasing blood flow to the infarcted region is to provide a route for circulating stem cells to seed and proliferate in the infarct region. Angiogenesis may lead to increased oxygenation for the surviving cellular islets within the infarct region, or to prime the infarct region for subsequent cell transplantation of myocardial regeneration. In the border zone, surviving cells would also benefit from an increase in blood supply through an angiogenesis process. In the remote zone, where cardiac cells tend to hypertrophy and become surrounded with some interstitial fibrosis, the ability of cells to receive oxygen an therefore function to full capacity are also compromised; thus, angiogenesis would be beneficial in these regions as well.
Bioscaffoldings have been formed from a number of different materials. One type of material is a pure alginate scaffolding. Alginate scaffoldings have been implanted as grafts containing fetal cardiac cells into rats and were shown to stimulate neovascularization and attenuated left ventricle dilation and failure. Alginate scaffolds are formed of algae, and are thus not a material that is found naturally in the body. Materials naturally found in the body, and in particular materials that are naturally found in the extracellular matrix (ECM) have also been used to form bioscaffoldings. The ECM is a complex network of fibrillar proteins and glycosaminoglycans, and serves to provide cells with information on their environment. Materials found in the ECM include collagen, hyaluronan, and laminin. The advantage of using these materials to form bioscaffoldings is that they are naturally occurring in the body and may therefore be degraded by enzymes naturally found in the body, such as hyluronidase and colleganase, and then absorbed. Non-functionalized bioscaffoldings formed of these materials include collagen/matrigel-based cardiac muscle constructs, alginate elastin, alginate laminin, hyaluronan, collagen, and collagen hyaluronan hydrogels. Hyaluronic acid hydrogel implants have been loaded with one of two cytokines, vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF), to elicit new microvessel growth in vivo. Some of these bioscaffoldings have been seeded with stem cells or other types of cells to generate new heart tissue and capillary growth into the scaffolding region. Providing non-functionalized scaffoldings alone to an infarct region of the heart can cause angiogenesis because it is a foreign body. But, non-functionalized scaffoldings or scaffoldings seeded with stem cells provide angiogenesis at a fairly slow rate. It would be valuable to provide a high rate of angiogenesis in as short a time as possible.